Heat transfer fluids are used in a variety of applications. One common use of heat transfer fluids is as a coolant in internal combustion engines. Most heat transfer fluids that are currently used contain water mixed with ethylene glycol (EG), a hazardous substance that can cause environmental contamination as a result of improper disposal. These fluids can cause dangerous health effects upon humans and other mammals if they are ingested. In addition, adverse health effects can occur due to exposure to used heat transfer fluids as a result of contamination by elemental heavy metal precipitates and toxic inhibitors that are added to prevent water related reactions.
Every year nearly 700 million gallons of heat transfer fluid concentrates are sold in the United States alone, and about 1.2 billion gallons are sold worldwide. Concentrates are formulations to which a substantial water fraction is added to form the actual heat transfer fluid. Much of the heat transfer fluid made from these concentrates replaces similar but spent heat transfer fluid drained from heat transfer systems such as automobile engine cooling systems. It is estimated that a significant percentage of the concentrates are disposed of improperly, resulting in contamination of the environment. Improper disposal by consumers is a major cause of this environmental contamination. Another major source of environmental contamination is leakage, spills and overflows from heavy duty vehicles. Experience with heavy duty vehicles shows that it is common to lose 10% of the engine heat transfer fluid volume after every 12,000 to 18,000 miles of operation due to leaks in the system components, such as the water pump, hose clamps or radiator core. This rate of loss is equal to about one gallon/month for the typical highway truck, which is the equivalent of a leakage rate of one drop per minute. A heat transfer fluid leak rate of one drop per minute is likely to go unnoticed, but can in total add up to a significant loss.
In some operations using heavy duty vehicles, overflows account for far more heat transfer fluid loss than low level leaks at the water pump, hose clamps or radiator core. Overflows occur due to overheating or when an engine cooling system is overfilled. When an engine cooling system is overfilled, operation of the engine heats the heat transfer fluid, causing expansion of the fluid that cannot be contained in the system. Pressure relief valve lines typically allow excess fluid to escape to the ground. Small spills and leaks (less than a gallon) of heat transfer fluid eventually will biodegrade with little impact to the environment. However, before biodegradation occurs, these spills and leaks can present a toxic danger to pets and wildlife.
Current engine coolant formulations typically utilize water as the primary heat removal fluid. The water content of an engine coolant is typically 30% to 70% by volume, depending upon the severity of the winter climate. The second major component of a conventional engine coolant is a freeze point depressant. The freeze point depressant most frequently used is EG, which is added to water in a range from 30% to 70% by volume of the engine coolant to prevent freezing of the water during winter. EG is a polyhydric alcohol, an alcohol with more than one hydroxyl (OH) group. Many polyhydric alcohols (such as, for example, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, diapropylene glycol and hexylene glycol) when added to water depress the freezing point of the water and elevate the boiling point of the water. The most commonly used polyhydric alcohol in engine coolant formulations is EG because it has excellent characteristics for that purpose and because it is the least expensive of the polyhydric alcohols.
In addition to water and EG, an additive package containing several different chemicals is included. These additives are designed to prevent corrosion, cavitation, deposit formation and foaming, and are each present usually in concentrations of from 0.1% to 3% by weight of the coolant concentrate. The additives are typically mixed with the freeze point depressant to form an antifreeze concentrate, which can be blended with water to form the engine coolant. As an alternative to EG, a formulation composed of the polyhydric alcohol propylene glycol (PG) with additives has been used as a freeze point depressant, primarily due to PG's lower toxicity rating as compared to EG.
The same coolant formulations are not used for all engines because different engine types have different requirements. For example, heavy-duty engines require a high concentration of sodium nitrite as an additive to control iron erosion of cylinder liners due to cavitation. Cylinder liner cavitation can occur when a substantial portion of the engine coolant is made up of water. When, for example, a mixture of 50% water and 50% EG is used (50/50 EG/W) in a heavy duty engine, the operating temperature of the coolant (about 200° F., 93.3° C.) is fairly close to the boiling point of the coolant (about 250° F., 121.1° C. at 10 psig). Vibration of the cylinder liner creates a low pressure area during the part of the cycle when the liner moves away from the coolant and a high pressure area when the liner moves toward the coolant. During the low-pressure part of the cycle coolant becomes vaporized, only to immediately collapse back to liquid during the high pressure part of the vibration cycle. The repeated high frequency formation and collapse of coolant vapor attacks the surface of the liner, eroding small amounts of iron. Sodium nitrite is added to limit the amount of vapor impacting the cylinder wall. By comparison, the use of sodium nitrite is not necessary or desirable in light duty engines. The complexity of balancing various water to EG (or PG) ratios and different additive formulations can result in improper freeze protection and clogged radiators and heater cores when the engine coolant is misformulated. As discussed further below, many of these problems are a result of the need for a substantial water fraction in these engine coolants.
Another difference between heavy-duty engines and light duty automobile engines is the use of supplemental coolant additives in heavy duty engines to replenish additives that are depleted with service. Supplemental coolant additives are not used or required in passenger cars that have a coolant life of 20,000 miles (32,186 km) to 30,000 miles (48,279 km). Heavy-duty service usually demands 200,000 miles (321,860 km) to 300,000 miles (482,790 km) before coolant replacement. The longer coolant service requirement results in the need to periodically replenish the inhibitors in heavy-duty engine coolants. Examples of commonly used supplemental coolant additives include sodium nitrite, dipotassium phosphate, sodium molybdate dihydrate, and phosphoric acid.
Supplemental coolant additives must be chemically balanced with the coolant volume, which can be difficult and costly to control properly. Improper balancing of additives can result in severe damage to cooling system components and the engine. If the concentration of the supplemental coolant additives in the coolant is too low, corrosion and cavitation damage to the engine and cooling system components can occur. If, on the other hand, the concentration of supplemental additives is too high, additives can precipitate from the coolant solution and clog radiator and heater cores. A further concern with supplemental coolant additives is that they may, under certain conditions, be difficult to properly dissolve in the engine coolant. If the supplemental additives do not completely dissolve, they may be a source of additional clogging problems in the engine.
Glycols make up 95% by weight of conventional antifreeze/coolant concentrates, and after blending with water, about 30% to 70% by volume of the coolant used in the vehicle. Because of its relative abundance and lower cost as compared with alternative glycols, conventional antifreezes are almost always formulated with EG. A major disadvantage of using EG as a freezing point depressant for engine coolants is its high toxicity to humans and other mammals if ingested. Toxicity is generally measured in accordance with a rating system known as the LD50 rating system, which is the amount of a substance expressed in milligrams per kilogram of body mass that, when fed to laboratory rats in a single dose, will cause the death of 50 percent of the laboratory rats. A lower LD50 value indicates a higher toxicity (i.e., smaller amounts of the substance can be lethal). An LD50 value of less than or equal to 5,000 milligrams per kilogram of body mass (mg/kg) can classify an antifreeze concentrate as hazardous. Because EG has an LD50 value of 4,700 mg/kg, EG is considered hazardous by this rating system. Moreover, EG is a known toxin to humans at relatively low levels.
The toxicity associated with EG is caused by the metabolites of EG, some of which are toxic. EG, when ingested, is metabolized by the alcohol dehydrogenase enzyme (ADH), converting it to glycoaldehyde. Glycoaldehyde further metabolizes to glycolic acid (glycolate). The accumulation of glycolic acid causes metabolic acidosis. Also, glycolic acid accumulation correlates with a decrease in arterial bicarbonate concentration. Some of the glycolic acid metabolizes to glyoxylic acid (glyoxylate), which further metabolizes to oxalic acid (oxylate). Oxalic acid binds to serum calcium in the bloodstream, and precipitates as crystals of calcium oxalate.
Characteristic symptoms observed with EG ingestion include anion gap metabolic acidosis, hypocalcemia, cardiac failure, and acute oliguric renal failure. Calcium oxylate crystals in many cases can be found throughout the body. Calcium oxylate crystals in the kidneys cause or are associated with the development of acute renal failure.
As reported in Toxic Release Inventory Reporting; Notice of Receipt of Petition, Federal Register, Vol. 63, No. 27, Feb. 10, 1998, the lethal dose of EG for a human is approximately 1,570 mg/kg body mass. Consequently, EG is classified by many regulatory authorities as a dangerous material. EG also has the added complication of a sweet smell and taste thereby creating an attraction for animals and children.
Due to the toxicity of EG, in recent years a base fluid concentrate with about 95% PG and additives has been used as a substitute for EG with additives in many antifreeze formulations. PG has an LD50 value of 20,000 mg/kg as compared to EG's 4,700 mg/kg. PG is considered essentially non-toxic, and it has been approved by the U.S. Food and Drug Administration as a food additive. One impediment to more widespread usage of PG as a base fluid for antifreeze concentrates is its relatively high cost as compared to EG.
All conventional antifreeze concentrates, whether EG or PG based, contain water in their formulations. EG antifreeze concentrates require a small percentage of water in their formulation because EG, by itself and without any water, freezes at +7.7° F. (−13.5° C.). A small amount of water must be added to depress the freezing point. Addition of four percent water by volume to EG lowers the freezing point of the mixture to 3° F. (˜19.4° C.). The freezing point of PG (by itself and without water) is relatively low, −76° F. (−60° C.). However, because some of the required additives are not readily soluble in either EG or PG, water is added to all conventional concentrate mixtures. Three to five percent by weight water is typically included in coolant concentrates to dissolve certain additives that will not dissolve in glycols. Added water is essential in conventional concentrates to keep the additives dissolved, particularly as the concentrates may be stored for extended periods.
Although three to five percent water is intentionally added to EG and PG concentrates to dissolve water soluble additives, addition of water alone is not effective over long periods of time to maintain the additives in solution. For long term storage, conventional coolant concentrates must be agitated periodically in order to keep the additives in solution until blending of the concentrate with water to make the final coolant mixture. If stored too long as a concentrate (over 6-8 months), one or more of the additives may precipitate from the solution and accumulate in the bottom of the container, forming a gel. The gelled additives will not return to solution, even with agitation. Even when mixed with water in an engine coolant, for example as 50/50 EG/water, the water soluble additives can form a gel if not agitated regularly by running the engine. This can be a severe problem for engines used in stationary emergency pumps and generators as well as military and other limited use engines.
The water added to concentrates to form an engine coolant can also cause formation of potentially hazardous products. Water at elevated temperatures can be highly reactive with the metal surfaces in a cooling system. The water can react with lead and copper materials from radiators, including brass and lead solder. As a result, precipitates of heavy metals, such as lead and copper, can become suspended in the circulating coolant in the engine. Water is also highly reactive with light alloys, such as aluminum, and the water fraction of the coolant can generate large amounts of aluminum precipitates, particularly at higher coolant temperatures. Even with the addition of additives to control these reactions, aluminum is constantly lost to the conventional engine coolants containing approximately 50/50 mixtures of EG and water.
Corrosion of metal surfaces in engine cooling systems using conventional glycol/water coolants is also caused by the formation of organic acids in the coolant, such as pyruvic acid, lactic acid, formic acid, and acetic acid. Polyhydric alcohols, such as EG or PG, in aqueous solutions can produce acidic oxidation products when in the presence of hot metal surfaces, oxygen from either entrapped air or water, vigorous aeration, and metal ions which catalyze the oxidation process. Moreover, formation of lactic acid and acetic acid is accelerated in coolant solutions at 200° F. (93.3° C.) or above while in the presence of copper. Formation of acetic acid is further accelerated in the presence of aluminum in coolant solutions at 200° F. (93.3° C.) or above. These acids can lower the pH of the coolant. Among the metals and alloys found in engine cooling systems, iron and steel are the most reactive to solutions containing organic acids, whereas light metals and alloys, such as aluminum, are considerably less reactive.
To counteract the effect of organic acids, conventional EG or PG based concentrates include buffers in their formulations. The buffers act to maintain the pH of the engine coolant in the range of approximately 10 to 11 as organic acids are formed. Some examples of typically utilized buffers include sodium tetraborate, sodium tetraborate decahydrate, sodium benzoate, phosphoric acid and sodium mercaptobenzothiazole. These buffers also require water in order to enter into and remain in solution. As the buffers in the coolant solution become depleted over time, the water fraction of the coolant reacts with the heat, air and metals of the engine, and, as a result, the pH decreases because of the acids that form.
In addition to buffers, all currently used and previously known engine coolants require inhibitors to control the corrosive effects from the water content of the coolant. The inhibitors must be balanced to avoid interactions with each other that would decrease their individual effectiveness. For example, phosphates and borates can decrease the corrosion protection provided to aluminum by silicates. Moreover, the inhibitors must not be used in excess concentration (in an attempt to extend the depletion time) because that can cause damage to system components due to precipitation resulting in plugging of radiator and heater core tubes. In addition, silicates, silicones, borates and phosphates are chemically abrasive and can erode heat exchanger tubes and pump impellers. Nevertheless, the inhibitors must still exist in a concentration adequate for protecting all of the metals.
All currently used coolant formulations require the addition of water to solubilize additives used as buffers, corrosion inhibitors and anti-foam agents. In addition, these water soluble additives require heat, extreme agitation, and extensive time for the water to react and cause the additives to dissolve. These requirements add significant cost and complexity to the formulation and packaging of antifreeze concentrates. The energy costs and time required for blending, before packaging, are a major factor in the processing costs. Also, because many of these additives may interfere with each other and cause an incomplete solution and failure of the formulation process, the formulating process must be monitored constantly to assure a proper blend.
Thus, the additive package that is included in known coolant concentrate formulations can consist of from 5 to 15, and typically from 8 to 15, different chemicals. These additives are broken down into major and minor categories, depending upon the amount used in an engine coolant formulation:
MAJOR (0.05% to 3.0%)MINOR (<0.05%)BufferDefoamerCorrosion inhibitorsDyeCavitation inhibitorsScale inhibitorSurfactantChelates
In addition, some of the additives themselves, e.g., borates, phosphates, and nitrites, are considered toxic. Thus, not only do all known coolant concentrate formulations include additives that require heat, extreme agitation and extensive time for the water to react and cause the additives to dissolve, but the additives themselves are sometimes toxic. Further, the additives require complex balancing which accommodates the prevention of interference between the additives, while also preventing the excessive presence of any one additive in the coolant.
The applicant has a co-pending application U.S. Ser. No. 08/991,155 filed on Dec. 17, 1997, which is continuation-in-part of patent application U.S. Ser. No. 08/409,026 filed on Mar. 23, 1995, the contents of each of which are expressly incorporated herein by reference.
Accordingly, it is an object of the present invention to overcome one or more of the drawbacks and disadvantages of the prior art and provide a reduced toxicity, non-aqueous heat transfer fluid.